(1) Field of the Invention
This invention relates to a switching power source device for supplying a desired AC/DC output by producing a pulse width-modulated waveform through ON-OFF control of switching elements serially connected to an input power source. The device eliminates high-frequency components of the pulse width-modulated waveform with a smoothing choke (hereinafter this operation will be referred to as "smoothing").
This invention particularly relates to a switching power source device which, in a system designed to control the output by alternatively switching a plurality of switching elements connected in pairs as in push-pull connection or bridge connection, precludes a current/voltage surge induced in consequence of the on/off control of the switching elements by dividing the smoothing choke into two portions. This device can be efficiently utilized in an AC uninterruptable power source system, a controller drive system for motors, a DC constant voltage power source system, etc.
(2) Description of the Prior Art
The switching power source device has the virtues of small size and high efficiency, and therefore, finds extensive utility as a power source device in many fields such as data processing systems. The power source devices having a relatively large power capacity, and the AC power source devices designed to derive a sine-wave AC output from a DC input power source, are among the power source devices of this principle. For those power source devices which are adapted to effect control of output power by alternately switching a plurality of switching elements connected in pairs, the push-pull connection or bridge connection are employed more often than not.
FIG. 1 is a schematic structural diagram illustrating a typical switching power source device of the conventional principle ("Power Semiconductor Circuits," pages 357-358, written by S. B. Dewan & A. Strengthen and published John Wiley & Sons, 1975). FIG. 2 is a waveform diagram illustrating the operating principle of the power source device in FIG. 1.
As illustrated in FIG. 1, a pair of DC power sources 51, 52 are serially connected to each other and a pair of switches 1, 2 are serially connected to the opposite terminals thereof. A choke coil 3 is inserted between a connecting node 7 of the switches 1,2 and an output terminal 10. A capacitor 4 is inserted between a connecting node B of the DC power sources 51 52 and the output terminal 10. A load 6 is connected with capacitor 4 in parallel between the node 8 and the output terminal 10. To the switches 1, 2, feedback diodes D1, D2 are respectively connected in parallel so as to be reverse biased by the power sources 51, 52.
During the operation of the switching power source device, when the switch 1 and the switch 2 are alternatively turned ON and OFF and the time ratio thereof is controlled in the form of a sine wave as illustrated in FIG. 2 (a), a voltage of rectangular waveform having the pulse width modulated as illustrated in FIG. 2 (a) is generated at the node 7. When the voltage of this rectangular waveform is deprived of high-frequency components with a filter (LPF) which is formed of the choke coil 3 and the capacitor 4, an AC output of sine waveform as illustrated in FIG. 2 (b) is obtained at the output terminal 10. This output is applied to the load 6.
Here no problem would arise if the switches 1, 2 are ideal switching elements and the signals for effecting ON-OFF control of these switches are ideal rectangular waveforms. In the actual device, however, various problems are posed because of characteristic properties inherent in the switching elements. The problems will be discussed below.
FIG. 3 illustrates working examples of the switches 1, 2 illustrated, in FIG. 1. The symbols used in FIG. 3 which are the same as those used in FIG. 1 denote the same features in each.
FIG. 3 (a) represents a case using bipolar transistors as switching elements. In this case when a signal for turning OFF transistor 1 is given to the base thereof and signal for turning ON a transistor 2 is given to the base thereof for causing a first state having transistor 1 in the ON status and transistor 2 in the OFF status, a reverse of these signals will cause a second state having the transistor 2 conversely in the ON status and, the transistor 1 in the OFF status. There is the possibility in doing this switching that the transistor 1 which has been in the ON status will be delayed by the storage time thereof in responding to the turn-off signal and, as a result, the two transistors 1 and 2 may both assume the ON status.
In this state, the power sources 51, 52 in FIG. 1 would be short-circuited via the transistors 1, 2 possibly to induce flow of unduly large current through the two transistors and cause breakage thereof. The storage time, as widely known, is the duration in which the transistor is forced by the excess carriers remaining in the base thereof to retain the ON status even after the supply of base current is cut off in response to the turn-off signal.
Numerous devices of the prior art allow for dead time (the duration in which the two transistors both remain in the OFF status) so as to preclude the two transistors from assuming the ON status at the same time ("Mospower Applications Handbook", pages 5-87 to 5-88, published by Siliconix Incorporated in 1984). Since the storage time is variable with the magnitudes of load current and ambient temperature, for example, it is not easy to attain accurate control of the dead time.
FIG. 3 (b) illustrates the case of using MOSFET's as switching elements. In this case, there is no possibility of the two FET's both assuming the ON status at the same time because of storage time due to residual carriers. However, since the static capacity between the drain and the source is large (ranging generally from hundreds to thousands of pF), there is the possibility that when one of the FET's is turned ON, the FET's both may assume the ON status at the same time because of a large dv/dt ratio (sharp change of voltage) occurring between the drain and the source of the other FET. As the result, the power sources are short-circuited by these FET's to permit flow of a surge current.
FIG. 3 (c) illustrates a case using GTO's (gate turn-off thyristors) as switching elements. In such switching elements as GTO's which possess self-retaining characteristics, there is the possibility that when one of the two switching elements is turned ON, the other switching element may be compelled to permit flow of an anode current by the dv/dt occurring between the opposite terminals thereof. This anode current partly finds its way to the gate to turn ON the other switching element which has been in the OFF status and, thus, bring about the possibility of the two switching elements assuming the ON status at the same time.
In any switching power source device using a plurality of switching elements connected in pairs, it is an ideal as may be surmised from the description given above, that the turn-on action of one of the two switching elements in any of the pairs and the turn-off action of the other switching element of the pair take place at exactly the same moment.
Generally the switching elements fail to produce, such ideal switching actions as described above, and there is a duration in which the switching elements in a pair assume the ON status at the same time or, conversely, the OFF status at the same time. As the result, these switching elements incur a current surge and/or a voltage surge.
For the protection of the switching elements against the current/voltage surge, and for the preclusion of the occurrence of noise due to the surge, the devices of the prior art have been adapted to absorb the surge current and voltage through inserting saturable magnetic cores 81, 82 in series therewith, or connecting snubber circuits 71, 72 in parallel to the switching elements 1, 2 as illustrated in FIG. 4, each snubber circuit consisting of a resistor and a capacitor in series. Since these measures are incapable of completely preventing the phenomenon of surge, however, the relevant switching elements and consequently the power source devices using them still suffer from insufficient reliability. Further, since the power consumed by the surge-absorbing elements and the heat generated thereby are substantially proportional to the speed and number of switching actions involved, the devices using such surge-absorbing elements have a disadvantage that an increase in the switching frequency is obtained only with difficulty.